Electrolytic manganese



United States Patent ELECTROLYTIC MANGANESE No Drawing. Application September 3, 1953,

Serial No. 378,426

3 Claims. (Cl. 204-105) The present invention relates to an improved electrolytic manganese process and, more particularly, to a process employing electrolyte additives that control the formation of the deposited metal.

In the production of metallic electrolytic manganese, the form of the deposited metal is of the greatest importance since it influences the efliciency of the process, the purity of the deposited metal, and the ease with which the deposited metal can be stripped from the cathode.

If the deposited metal is coarsely granular, it may build up on the cathode in small nodules that grow radially forming a cauliflower-like structure from which electrolyte is not easily washed. Also, in this form the metal is not easily stripped from the cathode material (if this is not the same metal as being deposited), since the metal tends to come oif in individual nodules rather than sheets or plates. In some processes, the deposited metal is removed from the cathodes by flexing or vibrating the latter; if the deposit is nodular, it tends to flex with the cathode and, does not break loose.

Another difliculty encountered is treeing; that is, some nodules may grow at a much greater rate than the average, thus becoming much extended above the surface of the cathode deposit. In some cells this may make it mechanically difficult to remove the cathodes, or may upset current distribution (thus aggravating the condition). In addition, the nodules tend to be broken oil and lost during handling, decreasing metal recovery.

A further difficulty encountered is the influence of cathode metal structure on deposition time. Obviously, if cathodes must be changed frequently, the overall process efliciency is greatly reduced. As mentioned above, treeing of the cathode metal, once started, becomes selfpropagating and the extension or growth of the trees can be allowed to continue only to a limited extent before the cathode must be changed, even though the volume of deposited metal is far less than that physically permissible in the form of a thin, rectangular body. Therefore, any addition which will prevent treeing and maintain formation of a relatively flat, smooth deposit will greatly increase the efliciency of the overall electrowinning process.

Heretofore, various organic compounds, such as gums and glues, have been used in many processes to prevent the formation of nodular structures, but these are not equally eflective with all processes.

It is an object of the present invention to extend the deposition time and thus simplify and improve the efliciency of the process for the electrowinning of manganese by reducing the frequency at which cathodes must be removed, stripped, prepared and reinserted in the cell.

Another object of the invention is to produce electrolytic manganese metal of improved purity.

A further object of the invention is to produce electrolytic manganese metal that is sound, dense, less friable and of greater thickness than otherwise obtained, and, therefore, easier to handle and use.

The electrowinning of some metals is much more easily accomplished than others; copper, silver, and nickel may be electrodeposited without particular difliculty. One of the metals which is much more difiicult to obtain electrolytically is manganese which tends to deposit in irregular fashion and in nodular form, and is very prone to form trees. Prior practice for electrowinning of manganese was such that not more than a 25 to 30 hours deposition period could be achieved. If it was attempted to continue deposition beyond this length of time, the quality of the metal degenerated and became unacceptable.

By the process of this invention, high-quality manganese metal can be electrodeposited for periods of two to three times as long as is possible with prior processes.

The present invention comprises a process for electrowinning of manganese from an electrolyte composed essentially of manganese sulphate, ammonium sulphate, and boric acid which serves as a smoothing agent.

The addition of small quanitties of boric acid to the electrolyte has a very marked effect upon the character of the manganese metal deposited upon the cathode. By varying the quantity of boric acid added to the electrolyte the character of the deposit may be altered from soft to dense, hard, and highly-stressed metal. This effect is not a continuously progressive improvement in metal quality, however, since there is a limit below which no improvement is obtained and there is an upper limit above which it is difiicult to obtain an adherent deposit.

It has been found that the influence of boric acid is not beneficial at all levels of concentration when added to a manganese sulphate electrolyte of a fixed base composition. However, to a limited degree, the efiect of the boric acid is related to the composition of the electrolyte, and minor control of the effect can be obtained by altering the ratio of the ammonium sulphate concentration to the manganese concentration in the catholyte. At a given level of boric acid, if the ratio is decreased, the quality of the deposited metal is improved but the time during which such metal is produced is shortened. If the ratio is raised, the opposite effect is obtained. In addition, there are limits to this ratio beyond which the boric acid tends to become ineffective; for instance, at a catholyte, boric acid concentration of 3 grams per liter, desirable metal is obtained when the ratio of ammonium sulphate to the manganese concentration is 10 to l or less. If this ratio is higher, 3 grams of boric acid are ineffective and a larger quantity must be added in order to restore the effect. If the ratio is decreased below 9 to 1 at a 3-gramper-liter boric acid level, the deposit becomes more dense and hard, but the deposition rate will slow down and stop within a relatively short period of time due to precipitation of manganese.

It has been found that the most desirable ratio of ammonium sulphate concentration to manganese concentration in the electrolyte (catholyte) is approximately 10 to l; the preferred level of boric acid addition is within the range of 4 to 8 grams per liter.

As is well known in the art, a small amount of sulphur dioxide is added to the cell feed solution (in a concentration of about 0.1 to 1.0 gram per liter) to stabilize the solution and to increase the current efliciency.

Marked improvement in the quality of the deposited manganese metal has been produced by the addition of boric acid in the proper proportion defined hereinabove. In one example a catholyte containing about 12 g./l. manganese, 131 g./l. ammonium sulphate, 0.5 g./l. of sulphur dioxide and 4.0 g./l. boric acid (HsBOa) was continually produced in the cathode compartment of a diaphragm electrolytic cell employing 99% lead-1% silver alloy anodes and stainless steel cathodes. The electrolyte fed to the cell contained 32 to 34 g./l. of manganese, g./l. ammonium sulfate, 0.5 g./l. sulfur dioxide and 4.0 g./l. boric acid. The cathode current density employed was about 40 amperes per square foot and catholyte pH was about 8.5. Metallic manganese was deposited continuously for greater than 48 hours and was found to be solid, dense, and almost free of nodules. A test performed under similar conditions without the use of boric acid in the electrolyte resulted in the deposition of a soft nodular metal at the end of 30 hours. Depositions for longer than 30 hours would only have served to reduce the overall density of the deposited sheet.

What is claimed is:

1. In a process for the electrowinning of metallic manganese employing a diaphragm compartment cell and a catholyte comprising an aqueous manganese and ammonium sulphate solution containing an effective amount of sulphur dioxide, wherein the ratio of ammonium sulphate concentration to manganese concentration of said catholyte solution is between about 7 to 1 and 12 to 1, the improvement which comprises adding boric acid to the catholyte cell feed in an amount sufficient to result in a concentration of between 2.0 and 15.0 grams per liter boric acid in said catholyte which will result in the deposition of a more dense metallic manganese deposit free of nodules.

. 2. A process for the electrowinning of metallic manganese employing a diaphragm compartment cell comprising providing an aqueous manganese and ammonium sulphate solution, adding an effective amount of sulphur dioxide to said solution, adding boric acid to said solution in an amount sufficient to result in a concentration of between about 2.0 and 15.0 grams per liter to form cell feed, introducing said cell feed into the catholyte compartment of said cell and electrolyzing, While maintaining an ammonium sulphate to manganese concentration ratio in the catholyte of between 7 to 1 and 12 to 1, to form at the cathode a solid, dense deposit of metallic manganese substanitally free of nodules.

3. In the catholyte compartments of a multi-compartment diaphragm cell for the electrowinning of metallic manganese from manganese-containing solutions, a catholyte comprising an aqueous solution of manganese and ammonium sulphate having an ammonium sulphate to manganese concentration ratio of between about 7 to l and 12 to 1, an effective amount of sulphur dioxide, and a boric acid concentration of between about 2 and 15 grams per liter.

Koster June 13, 1950 Koster Mar. 27, 1951 

1. IN A PROCESS FOR THE ELECTROWINNING OF METALLIC MANGANESE EMPLOYING A DIAPHRAGM COMPARTMENT CELL AND A CATHOLYTE COMPRISING AN AQUEOUS MANGANESE AND AMMONIUM SULPHATE SOLUTION CONTAINING AN EFFECTIVE AMOUNT OF SULPHUR DIOXIDE, WHEREIN THE RATIO OF AMMONIUM SULPHATE CONCENTRATION TO MANGANESE CONCENTRTION OF SAID CATHOLYTE SOLUTION IS BETWEEN ABOUT 7 TO 1 AND 12 TO 1, THE IMPROVEMENT WHICH COMPRISES ADDING BORIC ACID TO THE CATHOLYTE CELL FEED IN AN AMOUNT SUFFICIENT TO RESULT IN A CONCENTRATION BETWEEN 2.0 AND15.0 GRAMS PER LITER BORIC ACID IN SAID CATHOLYTE WHICH WILL RESULT IN THE DEPOSITION OF A MORE DENSE METALLIC MANGANESE DEPOSIT FREE OF NODULES. 